<!DOCTYPE>
<html>

<head>
    <title></title>
    <style>
        .parent {
            display: flex;
            align-items: center;
            height: 600px;
            background: #ccc;
            height:100%;
        }
        .child{
            background: yellowgreen;
            width:100%;
        }
    </style>
</head>

<body>
    <div class="parent">
        <div class="child">asdfdf
            <br/>asdfadf</div>
    </div>
</body>

</html>